A main problem encountered in applications of this type is the ability to implement measurements in a harsh environment, for which the conventional sensor technologies are not usable because of the harsh environmental conditions encountered, whether these environmental conditions be thermal, vibratory, chemical, etc.
An approach used hitherto to carry out such a measurement in a difficult environment such as this is to place a transducer between the quantity to be measured and the measuring device, in such a way that at the end of the transducer the environmental conditions are compatible with the technology of the sensor.
The role of the transducer is thus to form a screen for certain physical components that are troublesome or incompatible with the sensor, while being a transmitter of the quantity to be measured.
But the use of this transducer presents a certain number of drawbacks, notably because of deformations, loss or addition of information to which the transducer may give rise, etc.
These impairments are characterized for example by imperfections in amplitude (lack of precision or saturation) and/or in the dynamic sector (limited bandwidth at low and/or at high frequency).
An example of such a measurement in a harsh environment relates to the measurement of the cylinder pressure in a motor vehicle engine.
The measurement of such a cylinder pressure in an engine is a new requirement. This involves measuring during the various cycles of the engine (intake, compression, combustion, exhaust), the variation in gas pressure so as to optimize control of injection and ignition to obtain better efficiency and thus to decrease pollution.
But, during the engine cycles, the temperature varies from ambient temperature when the engine is off, to several hundred degrees C, knowing that the flame in the cylinder is at about 1800° C. Now, a certain number of materials used for sensors do not withstand such temperatures. Such is for example the case with materials such as silicon, which are customarily used for such pressure measurements.
It has thus been proposed to include the cylinder pressure sensor in a preheater sparkplug, for example for diesel engines. The tip of the sparkplug is then situated in the top of the cylinder in contact with the flame. It is used as transducer to compress at the other end of the sparkplug a sensor which is situated in a temperate environment in contact with the engine cylinder-head cooled by water circulation for example.
But the transduction performed by the tip of the preheater sparkplug is subject to mechanical inertia (mass of the transducer) and, according to the devices used, poses leaktightness problems, and also problems of precision in the low pressure values and/or is sensitive to vibrations.
A variant embodiment of this assembly also consists in including the cylinder pressure sensor in the preheater sparkplug, but differs from the previous solution in that the fixing device gives support to the pressure transducer on either side of the thickness of the cylinder-head. A deflection of the cylinder-head under the effect of the pressure is then measured.
But this measurement depends on the torque under which the preheater sparkplug is tightened in the cylinder-head. Moreover, it picks up the modes of vibration of the cylinder-head which are superimposed on the useful signal. Finally, the measured signal is affected by the transfer function which depends on the cylinder-head and its environment.
Another solution consists in using one or more optical fibres to pick up the reflection of a light ray by a membrane subjected to pressure and temperature.
The optical fibre then transports this information to a temperate place where signal acquisition and processing electronics are located.
However, the optical fibre also has temperature withstand limits. The membrane on which the light ray is reflected determines the quality of the measurement. Its definition must take account of the mechanical (natural mode of vibration) and thermal (expansion deforming the membrane) characteristics, problems of mechanical aging of the membrane, problems of oxidation on its reflecting face, etc.
Moreover, telecommunication technologies that make it possible to communicate data and/or to remotely power associated electronic devices are now emerging.
Thus for example, such means are used for tire pressure measurement.
Indeed it is difficult to run electrical links in a rotating part. A pressure sensor tied to the tire then experiences difficulties in communicating with the vehicle because of its displacements with respect to the latter.
A solution consists in including a self-powered sensor inside the tire and an emitter/receiver in the vehicle. The sensor can then communicate by a radiofrequency link with the emitter/receiver. In certain devices, the sensor can also be remotely powered by induction or by radiofrequency.
Such a structure can also be used to ensure temperature measurement in an oven.
Specifically, it is difficult to run electrical links in too hot an environment, the leads implementing for example solder joints, being sensitive to this type of environment, insofar as for example such solder joints may melt.
Radiofrequency communication means are also used in the automobile industry sector for vehicle antitheft facilities.
Thus for example, an identification device can be integrated into a motor vehicle key, this device not requiring any power supply in order to reduce maintenance to the minimum.
It is then possible to include in the key a device which in response to a radiofrequency signal returns a carrier signal bearing specific information according to the key. This device is then powered by virtue of the energy of the electromagnetic signal received.